Glass for multi-focal eyeglass lens

ABSTRACT

A glass for multi-focal eyeglass having a refractive index (nd) of 1.67 to 1.73, an Abbe number ( nu d) of 33 or more, a coefficient of thermal expansion of 82 to 95x10-7 and a softening point of 560 DEG to 640 DEG  C., comprising, in % by weight, 32 to 40% SiO2+B2O3+Al2O3; 22 to 40% SiO2; 0 to 10% B2O3; 0 to 4% Al2O3; 4 to 10% Li2O+Na2O+K2O; 0 to 4% Li2O; 0 to 7% Na2O; 0 to 7% K2O; 30 to 47% PbO; 3 to 24% BaO+SrO+CaO+MgO+ZnO; 0 to 20% BaO; 0 to 15% SrO; 0 to 10% CaO; 0 to 8% MgO; 0 to 15% ZnO; and 1 to 7% ZrO2; wherein a weight ratio of alkali metal oxides/PbO ranges from 0.1 to 0.3.

FIELD OF THE INVENTION

The present invention relates to a glass composition suitable for a highrefractive index segment which can form a multi-focal eyeglass lens byfusion bonding to a major portion composed of a lightweight glass havinga medium refractive index.

BACKGROUND OF THE INVENTION

In recent years, a lightweight eyeglass lens having a medium refractiveindex prepared from a glass having a relatively high refractive indexn_(d) of 1.60 to 1.61 but having a relatively small specific gravity of2.5 to 2.6 has been commercially available. Such a lens has widely beenmade habitual use because lens is thinner and lighter than conventionallens having n_(d) of 1.523 and provides improved sense of beauty andimproved feeling of wearing. However, in the case that a fusion bondingtype multi-focal lens is prepared using as a main portion a lightweightglass having a medium refractive index as described above, there is aninconvenience that the conventional glass for segment cannot be used.This is for the reasons that in the case of preparing a fusion bondingtype multi-focal lens, it is generally required that the coefficient ofthermal expansion of the segment be approximated to that of the mainportion to prevent cracking after fusion bonding, and the softeningpoint of the segment be set to a temperature slightly lower than thesoftening point of the main portion so as not to cause distortion of themain portion upon fusion bonding but the conventional glass for segmentdoes not necessarily satisfy such requirements.

Therefore, the glass for segment having thermal properties suitable fora lightweight glass having a medium refractive index, namely, having acoefficient of thermal expansion of 82 to 95×10⁻⁷ and a softening point(Littleton Point) of 560° to 640° C., and having a high refractive indexn_(d) of 1.67 to 1.72 is demanded.

A glass which almost satisfies those requirements is, for example, SF 8(coefficient of thermal expansion=91×10⁻⁷, softening point=595° C.,n_(d) =1.68893) or SF 15 (coefficient of thermal expansion=90×10⁻⁷,softening point=595° C., n_(d) =1.69895) introduced in catalogue (1980version) by Schott & Gen (West Germany). However, these glasses involvea disadvantage that their chromatic aberration is large due to theirlarge dispersion (ν_(d) =31.18 and 30.07, respectively). Accordingly, alarge Abbe number ν_(d) is also the requirement necessary in the glassfor segment.

In order to obtain a glass having a high refractive index, it isgenerally known to increase the proportion of components having highatomic valency of trivalent or more. As a result, however, the softeningpoint of the glass increases and the coefficient of thermal expansiondecreases. Conversely, the proportion of alkali metal oxides isincreased in order to reduce the softening point of the glass andincrease the coefficient of thermal expansion, but the refractive indexdecreases. Alkaline earth oxides impart middle properties betweencomponents of high atomic valency and alkali metal oxides. On the otherhand, PbO decreases a softening point and reduces a coefficient ofthermal expansion while it imparts a high refractive index.

SUMMARY OF THE INVENTION

As a result of extensive investigations on combinations of variouscomponents described above, it has been found that a glass having thedesired properties can be obtained by mixing alkali metal oxides andalkaline earth oxides or ZnO with PbO and strictly restricting theproportional relation of these components, and the present invention hasbeen accomplished.

Accordingly, an object of the present invention is to provide a glassfor segment having properties of a coefficient of thermal expansion of82 to 95×10⁻⁷, a softening point of 560° to 640° C., n_(d) of 1.67 to1.73 and ν_(d) ≧33 and which is capable of forming a glass formulti-focal eyeglass by fusion bonding to a main portion composed ofcommercially available, lightweight glass materials having a mediumrefractive index. No glass having such properties has been knownheretofore.

DETAILED DESCRIPTION OF THE INVENTION

The glass of the present invention has a composition comprising, in % byweight,

32 to 40% SiO₂ +B₂ O₃ +Al₂ O₃ ;

22 to 40% SiO₂ ;

0 to 10% B₂ O₃ ;

0 to 4% Al₂ O₃ ;

4 to 10% Li₂ O+Na₂ O+K₂ O;

0 to 4% Li₂ O;

0 to 7% Na₂ O;

0 to 7% K₂ O;

30 to 47% PbO;

3 to 24% BaO+SrO+CaO+MgO+ZnO;

0 to 20% BaO;

0 to 15% SrO;

0 to 10% CaO;

0 to 8% MgO;

0 to 15% ZnO; and

1 to 7% ZrO₂ ;

wherein the weight ratio of alkali metal oxides/PbO=0.1 to 0.3, and hasthe properties of n_(d) =1.67 to 1.73, ν_(d) ≧33, a coefficient ofthermal expansion=82 to 95×10⁻⁷ and a softening point=560° to 640° C.The present invention embraces not only a glass for segment but also amulti-focal eyeglass lens composed of the glass in combination with alightweight glass having a medium refractive index.

The components constituting the glass of the present invention isexplained in detail below. All percents herein are by weight.

The total amount of SiO₂, B₂ O₃ and Al₂ O₃ is limited to 32 to 40% inorder to obtain a refractive index of the desired range. Further, inorder to obtain a desired softening point, SiO₂ can be replaced by B₂ O₃and Al₂ O₃ up to 10% and 4%, respectively. However, the replacement inlarger amounts is not preferred since such results in deterioration ofchemical durability and resistance to devitrification. In the case thatSiO₂ is replaced by B₂ O₃ and Al₂ O₃, the amount of SiO₂ can be reducedto 22%.

LiO₂, Na₂ O and K₂ O are contained in amounts up to 4%, 7% and 7%,respectively. If the total amount thereof exceeds 10%, the coefficientof thermal expansion becomes excessively large and the softening pointbecomes too low. Further, if the total amount thereof is less than 4%,the coefficient of thermal expansion becomes too small.

PbO requires at least 30% in order to obtain a desired high refractiveindex, but if the amount thereof exceeds 47%, a desired Abbe numbercannot be maintained.

Further, in order to obtain the desired coefficient of thermal expansionand softening point, the weight ratio of alkali metal oxides/PbO shouldbe in a range of 0.1 to 0.3.

Furthermore, in order to adjust the coefficient of thermal expansion,softening point and Abbe number to the desired ranges, the total amountof BaO, SrO, CaO, MgO and ZnO should be in a range of 3 to 24%. BaO,SrO, CaO, MgO and ZnO result in poor resistance to devitrification uponfusion bonding if the amounts of these components exceed 20%, 15%, 10%,8% and 15%, respectively.

Further, in order to obtain the desired coefficient of thermal expansionand softening point, the weight ratio of alkaline earth oxides to PbO,alkaline earth oxides/PbO, should be between 0.2 and 0.6.

ZrO₂ contributes to improve the chemical durability and it isadvantageous to incorporate it in an amount of 1% or more but if theamount thereof exceeds 7%, devitrification resistance rapidlydeteriorates. In addition, the spirit of the present invention is notaffected by replacing La₂ O₃, TiO₂, etc., for a part of ZrO₂ or byincorporating As₂ O₃, Sb₂ O₃, F, etc., for purposes of purification,decolorization, etc.

Examples of the present invention are shown in the following table,wherein respective components are expressed in % by weight.

    __________________________________________________________________________               Example No.                                                        Component  1   2   3   4   5   6   7   8   9   10                             __________________________________________________________________________    SiO.sub.2  38.0                                                                              38.0                                                                              36.2                                                                              33.0                                                                              38.0                                                                              33.0                                                                              33.0                                                                              33.0                                                                              33.0                                                                              23.0                           B.sub.2 O.sub.3                                8.0                            Al.sub.2 O.sub.3                               2.0                            Li.sub.2 O     2.5 1.0     1.0 2.0 2.0 2.0 2.0 2.0                            Na.sub.2 O 4.8 1.2     5.2 4.2 1.2 1.2 1.2 1.2 1.2                            K.sub.2 O  2.4 1.5 4.2     3.0 2.0 2.0 2.0 2.0 2.0                            PbO        45.1                                                                              32.9                                                                              35.7                                                                              33.9                                                                              44.8                                                                              33.9                                                                              33.9                                                                              33.9                                                                              33.9                                                                              33.9                           BaO            13.2                                                                              12.2                                                                              13.2                                                                              4.0 13.2                                                                              3.2 5.2 8.2 13.2                           SrO                                10.0                                       CaO                                    8.0                                    MgO                                        5.0                                ZnO        4.7 4.7 9.7 9.7     9.7 9.7 9.7 9.7 9.7                            ZrO.sub.2  3.0 6.0 1.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0                            Alkali metal                                                                             0.16                                                                              0.16                                                                              0.15                                                                              0.15                                                                              0.18                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                           oxide/PbO                                                                     n.sub.d    1.680                                                                             1.683                                                                             1.679                                                                             1.705                                                                             1.677                                                                             1.712                                                                             1.710                                                                             1.718                                                                             1.710                                                                             1.721                          ν.sub.d 33.1                                                                              37.6                                                                              36.1                                                                              34.9                                                                              34.1                                                                              35.5                                                                              35.5                                                                              35.4                                                                              35.6                                                                              35.6                           Coefficient of thermal                                                                   90  86  84  89  94  88  91  92  87  91                             expansion (× 10.sup.-7)                                                 Softening point (°C.)                                                             590 630 610 630 570 620 610 630 630 590                            __________________________________________________________________________

Glasses described above can be prepared by roughly melting a mixture ofraw materials conventionally used such as siliceous stone powder, boricacid, aluminum hydroxide, lisage, barium carbonate, zinc flowers,zirconium oxide, etc., in a refractory crucible at 1,250° to 1,300° C.,transferring the melt into a platinum-made crucible, remelting at 1,300°to 1,350° C., stirring the melt to homogenize, removing bubbles, castinginto a mold previously heated at an appropriate temperature and thenannealing the product.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A glass for a multi-focal eyeglass having arefractive index (n_(d)) of 1.67 to 1.73, an Abbe number (ν_(d)) of 33or more, a coefficient of thermal expansion of 82 to 95×10⁻⁷ and asoftening point of 560° to 640° C., consisting of, in % by weight,32 to40% SiO₂ +B₂ O₃ +Al₂ O₃ ; 22 to 40% SiO₂ ; 0 to 10% B₂ O₃ ; 0 to 4% Al₂O₃ ; 4 to 10% Li₂ O+Na₂ O +K₂ O; 0 to 4% Li₂ O; 0 to 7% Na₂ O; 0 to 7%K₂ O; 30 to 47% PbO; 3 to 24% BaO+SrO+CaO+MgO+ZnO; 0 to 20% BaO; 0 to15% SrO; 0 to 10% CaO; 0 to 8% MgO; 0 to 15% ZnO; and 1 to 7% ZrO₂;wherein the weight ratio of alkali metal oxides/PbO ranges from 0.1 to0.3.